The use of image sensors is known in numerous applications ranging from the general-consumer gadgets sector, to the professional photography, to gaze tracking, and to industrial, medical and/or scientific uses, just to cite a few.
A typical image sensor comprises a plurality of pixels operatively connected to a control unit adapted to selectively bias said pixels and read them out. Each pixel includes a photo-active element or photodetector, which is usually a photodiode.
The image sensor market is at present dominated by active pixel sensors (APSs), which are fully-compatible with the CMOS process. A typical pixel in an APS comprises a photodiode for the collection of light, a switching element (such as for example a transistor) to allow the pixel to be individually addressed during readout, and an amplifier.
Current technology trends in the APS design aim at the miniaturization of the pixels while, at the same time, embedding more functionality in the pixels to provide enhanced features, such as for instance global shuttering or noise reduction among others. However, these conflicting trends complicate the design of the pixel and that of the overall image sensor.
As the size of the pixels shrinks, so does the size of their photodiodes. Given that the quantum efficiency of typical photodiodes cannot exceed one for the visible and infrared ranges, APSs critically rely on reaching very low noise levels and/or on using long exposure times, to achieve high signal-to-noise ratios. Moreover, as more and more transistors are required inside the pixel to implement such advanced functionality, the area available for light collection of the photodiode (or pixel fill factor) is further decreased. Therefore, image sensors with an improved pixel design and a more sophisticated readout circuit will be necessary to cope with the increasingly demanding performance specifications.
Back-side illuminated image sensors have been developed in an attempt to overcome the reduction in pixel fill factor of conventional image sensors (also referred to as front-side illuminated image sensors). In a back-side illuminated image sensor, the in-pixel readout electronics is arranged behind the semiconductor layer comprising the photodiode, as opposed to their front-side illuminated counterparts in which said in-pixel readout electronics lays on the same semiconductor layer as the photodiode or above. This is typically done by flipping the semiconductor wafer during manufacturing and then thinning its reverse side so that the incoming light can impinge on the photodiode without passing through the in-pixel readout electronics. Back-side illuminated image sensors achieve a substantial improvement in the pixel fill factor and, hence, in their photon-collecting ability, improvement which is even more significant when the pixel-size is small. However, one important shortcoming of back-side illuminated image sensors is that their manufacturing becomes dramatically more complicated and costly.
After APSs, the second largest portion of the market of image sensors is occupied by charged-coupled devices (CCDs) which, although also using a photodiode for light collection, their manufacturing and operation is quite different from that of APSs. In a CCD the charge generated by the collection of photons at a given pixel, and initially stored in a capacitive storage element in said pixel, is then transferred from within the device to a processing area where it can be converted to an electrical signal. Typically, the transfer of the photo-collected charge of the pixels to the processing area is done in a stepped and synchronized manner in which the charge collected in a pixel of each row (or column) of a two-dimensional arrangement of pixels is progressively shifted by one row (or column) and stored in the capacitive storage element of the pixel in the adjacent row (or column) until eventually reaching the processing area of the CCD.
Compared to APSs, CCDs do not require switching elements or amplifiers to be provided inside the pixel. However, one of the main drawbacks of this type of image sensors is that they need a more complex readout electronics to handle the charge shifting process. Moreover, CCDs require a dedicated manufacturing technology that is costly and, more importantly, incompatible with standard CMOS processing.
Another important aspect to take into account is the spectral range in which an image sensor is to operate as it will greatly determine the choice of the available light-absorbing materials for the fabrication of the photo-active element of the pixels.
In that sense, silicon is widely used in image sensors operating in the visible and near infrared ranges. In contrast, compounds such as InGaAs or HgCdTe, among others, are often employed for the infrared range (including short-wave infrared and/or long-wave infrared subranges). Finally, for image sensors operating in the ultraviolet region, and shorter-wave ranges, some known suitable materials include wide-gap semiconductors, such as for instance AlGaN.
Image sensors that integrate silicon (e.g., CMOS technology) for their control unit with photosensitive materials other than silicon for the photo-active elements of the pixels (also referred to as hybrid image sensors) offer an extended operating spectral range compared to CMOS-based image sensors. However, as for CMOS-based image sensors, hybrid image sensors do not provide a practical solution to the technological challenges of miniaturization and embedding more functionality at the pixel level, with the added disadvantage that such hybrid integration involves difficult and costly bonding processes.
The rapid development in the recent years of a market for consumer gadgets, wearable devices and mobile applications has stirred a growing interest in the development of technology able to provide components, and even full devices, being flexible and/or stretchable and/or transparent (or at least partially transparent) to the human eye.
Given that most of such devices incorporate image sensors, it would be desirable to have an imaging technology able to provide flexible and/or transparent image sensors. However, none of the imaging technologies described above is intended to produce image sensors with such properties.
Some image sensors have been proposed in an attempt to provide a transparent solution. For example, document U.S. Pat. No. 5,349,174 A discloses an image sensor having a two-dimensional arrangement of pixels disposed on a transparent substrate. In addition, the pixels of said image sensor comprise some elements, such as for instance a storing capacitor, that are also transparent. Although the resulting image sensor is semitransparent (as only a portion of the area occupied by the pixels is transparent), it is not intended to be flexible. Moreover, the control unit of the image sensor requires in-pixel switching elements for addressing individual pixels upon readout, which reduces the pixel fill factor and increases the complexity of the pixel design and that of the readout circuit of the control unit.
There have also been some attempts to provide a flexible image sensor. For example, document U.S. Pat. No. 6,974,971 B2 describes an image sensor that is bendable up to a certain extent, and that includes an array of pixels disposed on discrete areas of a substrate. Selected regions of the substrate, away from those areas in which the pixels are formed, are weakened to encourage flexing of the substrate to occur preferentially at those regions upon bending of the device and, in this manner, reduce the risk of damaging the pixels. Another example is disclosed in U.S. Pat. No. 8,193,601 B2, in which an image sensor comprises a plurality of pixels, each having a PIN photodiode as photo-active element, disposed on a flexible substrate. However, these solutions are far from satisfactory as in-pixel selection elements, in particular thin-film transistors (TFTs), are still required to selectively read out the pixels.
Photo-active elements based on organic photodiodes, as the ones described in U.S. Pat. No. 6,300,612 B1, have also been thought of as promising candidates for flexible and transparent image sensors. However, these image sensors will generally still need an in-pixel switching element for addressing individual pixels. Moreover, organic photodiodes have a fairly limited responsivity, well below 1 A/W, which might be problematic when used in image sensors, especially in those featuring small-sized pixels.
The use of active devices based on two-dimensional (2D) materials, such as for instance graphene, for different applications is the object of on-going research. For example, single-pixel photodetectors having a photosensitive element made of graphene have been demonstrated as proof of concept. The use of photodetectors based on 2D materials (e.g., graphene, as disclosed in for instance U.S. Pat. No. 8,053,782 B2) or on semiconductor nanocrystals (e.g. quantum dots, see for example U.S. Pat. No. 8,803,128 B2) in the pixels of full-size image sensors has also been proposed. However, such image sensors typically exhibit limited photoconductive gain.
Therefore, it would be highly desirable to have image sensors in which the photosensitive element of their pixels is capable of providing a high photoconductive gain, without compromising the pixel sensitivity due to, for example, high dark current levels.
Document US2014353471A1 describes a dark current suppression scheme based on a photosensitive and a shielded photodiode and which includes only one biasing circuit (providing bias voltage VRT, as shown in its FIG. 1). The scheme proposed in said document provides dark current compensation based on temperature information and temperature dependent calibration information.
Document WO 2013/017605 A1 discloses a phototransistor comprising a transport layer made of graphene, and a sensitizing layer disposed above the transport layer and that is made of colloidal quantum dots. The sensitizing layer absorbs incident light and induces changes in the conductivity of the transport layer to which it is associated. The high carrier mobility of graphene and the long carrier lifetime in the quantum dots make it possible for the phototransistor disclosed therein to obtain a large photoconductive gain. However, the device can only achieve desired responsivity levels at the expense of increased dark current levels, which in turn degrade the sensitivity and the shot-noise limit of the device.
Document US 2014/0299741 A1 refers to a transparent ambient-light sensor using sensitized graphene photodetectors that comprise two types of quantum dots arranged on a sheet of graphene. By detecting the difference in response of the two types of quantum dots, the sensor can provide ambient light and bandwidth sensing. Although this solution works for a reduced number photodetectors, it is not scalable to imaging applications involving a large number of pixels (typically a few millions), each pixel comprising a photodetector, as the power consumption of the device to bias simultaneously all the pixels would be prohibitive for any practical image sensor. Moreover, the architecture of the ambient-light sensor is very different from that of an image sensor, the latter requiring a control unit to selectively read out the pixels.
Paper ‘A CMOS image sensor with a double junction active pixel’, IEEE Transactions on Electron Devices, Vol. 50, no. 1, pp 32-42, by Findlater K. M. a t al., discloses a CMOS image sensor that employs a vertically integrated double-junction photodiode structure. Some elements of the read-out circuit of the image sensor disclosed in said paper are local, i.e. are arranged at the pixel level. Specifically, for the arrangement shown in its FIG. 7, the pixels contain six active transistors to which reset and read signal lines are connected, and which therefore constitute local elements of the read-out circuit. The photodiodes forming the image sensor disclosed in said paper cannot be considered as photosensitizing elements, and the arrangement forming the image sensor does not either comprise a transport layer for transporting electric charge carriers.
It is therefore an object of the present invention to provide an enhanced image sensor in which the integration of its pixels with the control unit can be done in a simple and efficient manner, while avoiding a reduction in the pixel fill factor due to in-pixel read-out electronics.
It is also an object of the present invention to provide an image sensor in which its pixels comprise an improved photo-active element capable of high photoconductive gain, i.e. a built-in photoconductive gain, and/or enhanced responsivity.
It is a further object of the present invention to provide an image sensor with an improved sensitivity of its pixels, and that does not require deep cooling of the device to achieve high signal-to-noise ratios.
It is yet another object of the present invention to provide an image sensor well-suited for flexible and/or stretchable and/or transparent optoelectronic devices.
It is yet another object of the present invention to provide a gaze tracking apparatus based on the image sensor of the invention.